Photovoltaic devices are known in the art (e.g., see U.S. Pat. Nos. 6,784,361, 6,288,325, 6,613,603 and 6,123,824, the disclosures of which are hereby incorporated herein by reference). Amorphous silicon (a-Si) and CdTe type photovoltaic devices, for example, each include a front contact or electrode.
Pyrolitic SnO2:F transparent conductive oxide (TCO) is often used as a front transparent electrode in photovoltaic devices. One advantage of pyrolitic SnO2:F for use as a TCO front electrode in photovoltaic devices is that it is able to withstand high processing temperatures used in making the devices. However, from the viewpoint of uniformity, potential cost savings, and film smoothness, pyrolytically deposited TCOs are not desirable. Thus, it will be appreciated that a sputter-deposited TCO for use as an electrode in a photovoltaic device would be more desirable with respect to one or more of uniformity, cost savings and/or film smoothness.
In certain example instances, it is possible for the front electrode of a photovoltaic device to be made of a transparent conductive oxide (TCO) such as tin oxide, zinc oxide (possibly doped with Al, i.e., ZnAlOx), or indium-tin-oxide (ITO) formed via sputtering on a substrate such as a glass substrate. However, in certain applications, such as CdTe photovoltaic devices as an example, high processing temperatures (e.g., 550-600 degrees C.) are used during manufacturing. High processing temperatures (e.g., 220-300 degrees C. or higher, with an example being about 250 degrees C.) may also be used in making a-Si and/or micromorph solar cells.
Unfortunately, conductive sputter-deposited TCOs such as ZnAlOx and ITO formed in a conventional sputtering process tends to lose significant amounts of electrical conductivity when heated to high temperatures (high temperatures may be needed in photovoltaic device manufacturing in certain instances). This loss of conductivity may be caused by fast oxygen migration from grain boundaries into the bulk of the crystallites. Moreover, at extremely high temperatures (e.g., 625-650 degrees C.), structural transformation of zinc oxide starts to occur.
Thus, it will be appreciated that there is a need in the art for techniques for making or forming a TCO electrode in a manner that improves the TCO's electrical conductivity as deposited and/or after high temperature processing.
It is known that lower sheet resistance in ITO films in comparison to its as-deposited counterpart typically requires activation. The activation, in turn, typically is achieved by baking in air in the temperature range of 300-350 degrees C. for 30-120 minutes. The inventors of the instant invention, however, have discovered that baking ITO films in a low oxygen environment such as a vacuum can further reduce the resistivity in comparison to air baked samples. Surprisingly and unexpectedly, the reduction in resistivity has been as much as 40% in certain example embodiments, when the deposited ITO thin film is baked in a vacuum or other low oxygen environment at a temperature elevated above the conventional activation temperature. For example, the inventors of the instant application surprising and unexpectedly have discovered that a significant reduction in resistivity may be achieved by baking a sample in a low oxygen or vacuum environment at a temperature of at least about 425 degrees C., more preferably at least about 450 degrees C., still more preferably at least about 475 degrees C., and most preferably at least about 500 degrees C. This activation technique of certain example embodiments also surprisingly and unexpectedly improves the optical properties of the activated ITO thin film.
Certain example embodiments of this invention relate to a method of activating an indium tin oxide (ITO) thin film deposited, directly or indirectly, on a substrate. The ITO thin film is baked in a low oxygen environment at a temperature of at least 450 degrees C. for at least 10 minutes so as to provide for (1) a post-baked resistivity of the ITO thin film that is below a resistivity of a corresponding air-baked ITO thin film, (2) a post-baked visible spectrum absorption and transmission of the ITO thin film that respectively are below and above the absorption and transmission of the corresponding air-baked ITO thin film, and (3) a post-baked infrared reflectivity of the ITO thin film that is above the reflectivity of the corresponding air-baked ITO thin film.
Certain example embodiments of this invention relate to a method of making a photovoltaic device. A substantially transparent substrate is provided. A layer comprising indium tin oxide (ITO) is disposed, directly or indirectly, on the substrate. The layer comprising ITO is heated in a low oxygen environment to a temperature of at least 475 degrees C. for at least 10 minutes so as to provide for (1) a post-baked sheet resistance of the layer comprising ITO that is below a sheet resistance of a corresponding air-baked ITO thin film, (2) a post-baked visible spectrum absorption and transmission of the layer comprising ITO that respectively are below and above the absorption and transmission of the corresponding air-baked ITO thin film, and (3) a post-baked infrared reflectivity of the layer comprising ITO that is above the reflectivity of the corresponding air-baked ITO thin film.
Certain example embodiments of this invention relate to a method of making an electronic device. A substrate having a layer comprising indium tin oxide (ITO) formed, directly or indirectly, thereon is provided. The layer comprising ITO is heated in a low oxygen environment to a temperature of at least 475 degrees C. for no more than 60 minutes to activate the layer comprising ITO, such that the layer comprising ITO has a sheet resistance lower than a sheet resistance of a corresponding layer comprising ITO activated in air-inclusive environment at a temperature no greater than 350 degrees C. The substrate is built into the electronic device.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.